The 20F2 screened RF pentode was designed for use in universal (ie no mains transformer) sets with a series connected heater chain. Mostly in television receivers at RF or as a sync separator.
The construction is all-glass with a metal skirt and location pip and spigot. The clear envelope reveals the screen inside. Inside the screen are the three wire grids and the anode is a pair of vertical plates joined by a strap.
The cathode is a rectangle and the control grid supports are made of copper with a cooling fin on top. The suppressor grid wire can be seen at the top of the shield (screen).
The thin glass tube envelope is 20 mm in diameter and, excluding the B8A base pins, is 53 mm tall.
References: Datasheet & 1040. Type 20F2 was first introduced in 1949.